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Message-ID: <1511366802.18523.119.camel@codethink.co.uk>
Subject: Re: [PATCH 4.14 00/31] 4.14.1-stable review
From:   Ben Hutchings <ben.hutchings@codethink.co.uk>
To:     Guenter Roeck <linux@roeck-us.net>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
        akpm@linux-foundation.org, shuahkh@osg.samsung.com,
        patches@kernelci.org, stable@vger.kernel.org
Date:   Wed, 22 Nov 2017 16:06:42 +0000
In-Reply-To: <20171121193845.GA9680@roeck-us.net>
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         <1511282778.18523.114.camel@codethink.co.uk>
         <20171121170914.GA21561@kroah.com>
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Organization: Codethink Ltd.
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On Tue, 2017-11-21 at 11:38 -0800, Guenter Roeck wrote:
> On Tue, Nov 21, 2017 at 07:07:14PM +0000, Ben Hutchings wrote:
> > On Tue, 2017-11-21 at 18:09 +0100, Greg Kroah-Hartman wrote:
> > > On Tue, Nov 21, 2017 at 04:46:18PM +0000, Ben Hutchings wrote:
> > > > On Tue, 2017-11-21 at 17:35 +0100, Greg Kroah-Hartman wrote:
> > > > > On Tue, Nov 21, 2017 at 03:26:10PM +0000, Ben Hutchings wrote:
> > > > 
> > > > [...]
> > > > > > Not all 32-bit configurations can provide cmpxchg64().  i40e's use of
> > > > > > cmpxchg64() appears to be fixed by:
> > > > > > 
> > > > > > b74f571f59a8 i40e/i40evf: organize and re-number feature flags
> > > > > > b48be9978e4b i40e: fix flags declaration
> > > > > 
> > > > > So without those patches, are any specific arches/configs broken for
> > > > > 4.14?
> > > > 
> > > > 32-bit parisc is.
> > > 
> > > Ok, but that's a horrid hack on the i40e driver, it just happens to move
> > > the bitfield to a 32bit variable.  Can't we just provide a "real"
> > > cmpxchg64() for 32-bit parisc?
> > 
> > No.  There is a generic implementation of cmpxchg64() but it is only
> > suitable for non-SMP configurations.
> > 
> 
> Dave implemented the following for sparc32 (in arch/sparc/lib/atomic32.c).
> 
> u64 __cmpxchg_u64(u64 *ptr, u64 old, u64 new)
> {
> 	unsigned long flags;
> 	u64 prev;
> 
> 	spin_lock_irqsave(ATOMIC_HASH(ptr), flags);
> 	if ((prev = *ptr) == old)
> 		*ptr = new;
> 	spin_unlock_irqrestore(ATOMIC_HASH(ptr), flags);
> 	return prev;
> }
> EXPORT_SYMBOL(__cmpxchg_u64);
> 
> Maybe something like this would work for other 32 bit architectures as well ?

Yes, you're right, and we even have generic code for this in
lib/atomic64.c - but only for atomic64_t, not u64.

Ben.

-- 
Ben Hutchings
Software Developer, Codethink Ltd.


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